Exothermic welds have long been recognized and the best electrical quality connection between two parts or conductors. No mechanical connection makes a comparable low resistance, high ampacity, long lived connection.
Exothermic welds are made using graphite as ceramic mold parts which form a crucible, a weld chamber, and an interconnecting tap hole. An exothermic reaction mixture is placed in the crucible on a fusible steel disc. When the exothermic material is ignited, the reaction produces molten metal which fuses the disc. The molten metal runs down through the tap hole and welds the parts to be welded in the weld chamber. Any slag formed rises to the top and enters the tap hole last to collect in an expanded section of the tap hole called a riser chamber. The slag is removed after the weld is formed.
After the weld is cooled, the molds are opened and cleaned for reuse. A mold set is usually opened and closed by one or more toggle frames which when closed, clamp the parts together to form the complete mold assembly.
Exothermic welds are used for a variety of connections which range from full fusion connections to "tack" welds. "Tack" welds are usually smaller charges and are widely used to "tack" a conductor to a surface or another object such as a ground rod, pipe, rebar, rail, or any major surface such as a steel sheet, column or beam.
Where the weld is to be formed on the side of a vertical object such as a ground rod or rebar, the crucible may be horizontally offset from the weld chamber and connected to the weld chamber by a dog leg tap hole, or one which has a significant bend or curvature to it.
A tap hole that cannot be readily cleaned usually requires three mold parts, two split along the axis of the hole, and both split from a third through the weld chamber. This makes the apparatus, and the assembly and opening of the mold parts complex, expensive, and awkward. Since mold parts and subassemblies are held together and opened and closed by toggle frames, three parts usually requires two frames which project from the molds. If other parts such as a clamp are used to hold the assembly to a workpiece or part, the entire assembly becomes overly complex and requires a considerable amount of space.
The economics of exothermic welding, whether the weld be a full fusion weld or simply a "tack" weld, dictate that the apparatus be as simple as possible and reusable. If preparation or limited life parts adds to the cost, then at some point the owner will sacrifice a quality electrical connection for cost.
Accordingly, it would be desirable to have a mold assembly for such welds having only two parts, greatly simplifying the assembly, but also one in which the tap hole can quickly be cleaned for reuse.